1. Field of the Invention
This invention relates to an alignment device for a mask and a wafer in an IC (integrated circuit) projection exposure apparatus. The projection exposure is the imaging type of exposure by a so-called optical lens using visible and ultraviolet light rays.
2. Description of the Prior Art
Generally, in the projection exposure method by the optical lens of a printing apparatus for manufacturing LSI or VLSI integrated circuits, the pattern of a mask is projected upon a wafer through an imaging projection lens. Particularly, in an apparatus wherein the mask pattern is projected and exposed upon the wafer while being reduced in scale to about 1/10, the same pattern is repetitively exposed on a single wafer. It is therefore desirable to detect, observe and confirm the relative positions of the mask and wafer through a projection lens each time the IC chip is printed. Such a system is called the step alignment or the each alignment (hereinafter referred to as the step alignment).
To carry out the above-described step alignment, for example, an observation optical system, as will be later described and as shown in FIG. 1 of the accompanying drawings, has been provided on the upper portion of the projection lens and the relative relation between the alignment mark on the mask and the alignment mark on the wafer has been confirmed through the observation optical system. A device of this type is generally called the alignment optical system or device.
However, problems in alignment arise, chiefly due to the poor contrast of the wafer pattern. That is, usually, the pattern on the wafer is of low contrast and particularly, in its exposed condition, the photoresist applied to the whole surface of the wafer causes lower contrast to occur.
For example, when an alignment mark pattern of SiO.sub.2 on a Si wafer is considered, the total reflection factors of Si and SiO.sub.2 are almost equal to each other, and it is therefore difficult to visually distinguish the mark because of low contrast. Accordingly, at present, in the case of a SiO.sub.2 alignment mark, patterns having minute periods are combined together and use is made of the phenomenon that the scattered light and diffracted light around each periodic pattern structure of the mark spread outside of the projection system; and when this is viewed as an image, the peripheral portion of the edge becomes a dark portion. However, even this still suffers from the disadvantage that the contrast is poor. Also, as a method of making an alignment mark, there is a method which increases the wafer printing step by one cycle and utilizes the anisotropic etching of the wafer to provide the wafer surface with a dug-in mark to thereby achieve increased contrast. However, this method suffers from the disadvantage that the steps of process are complex.
In view of this, there has recently been proposed a method which uses laser light as a light source, projects a spot upon SiO.sub.2 alignment marks through a projection lens and detects the scattered light from the mark. However, this method also suffers from the following disadvantages.
Firstly, the projection lens in which aberrations occur for laser light usually optimizes the aberrations for only one to two wavelengths of white illumination light. Secondly, a laser light source must be used in addition to an illumination light source and, therefore, during alignment the illumination light is wasteful. Also, the special need for the laser leads to the expensiveness of the device.